1. Field of the Invention
The present invention relates to a chromatic dispersion compensator for compensating chromatic dispersion in a transmission path of an optical fiber communication system and relates to a chromatic dispersion compensating optical communication system using such a chromatic dispersion compensator.
2. Description of the Related Art
If there arises chromatic dispersion in an optical fiber used as a transmission path in a optical digital transmission system, lowering of transmission quality is brought about. This is because chromatic dispersion and chirping produced by direct modulation of a semiconductor laser as a transmitter are coupled to generate waveform distortion. The degradation of transmission quality caused by waveform distortion becomes more remarkable as the bit rate increases. For example, in the case of 10 Gbps, it is necessary that the waveform distortion (spreading) is controlled to be sufficiently smaller than the time width of one slot, that is, about 10 ps which is one-tenth as much as 1/10 Gbps=100 ps.
As adding to the recent improvement of the bit rate, the distance of repeaterless transmission has been elongated with the advent of an Er-doped optical fiber amplifier having an amplifying function in a 1.55 .mu.m band in which a silica based optical fiber exhibits the lowest transmission loss, the necessity of delicately controlling the value of chromatic dispersion has arisen even in the case where a dispersion shifted fiber (DSF) having zero dispersion wavelength in the 1.55 .mu.m band is used as a transmission path. For example, there is used a chromatic dispersion compensating technique in which accumulated chromatic dispersion is cancelled by a fiber having chromatic dispersion with opposite sign at each relay point.
Further, in the case where a 1.55 .mu.m-band optical fiber amplifier is used in the transmission path of a 1.3 .mu.m-band single mode fiber (1.3 SMF) having been already installed or in the transmission path of a very low loss pure silica core fiber, large positive chromatic dispersion of these fibers in the 1.55 .mu.m band becomes a problem. Therefore, a dispersion compensating fiber having large negative chromatic dispersion in the 1.55 .mu.m wavelength band has been developed. Such a dispersion compensating fiber is known by ELECTRONICS LETTERS, Vol. 30, No. 2, (Jan. 20, 1994), pp. 161-162.
Considering further that the capacity will be increased more and more in the future, a wavelength division multiplexing transmission method (WDM) promises a bright future. In this case, it is necessary that chromatic dispersion takes zero in a wavelength range of an optical signal used. However, chromatic dispersion itself has dependency on wavelength. In the case of a matched cladding type fiber, the slope of dispersion, that is, the dispersion slope, is generally positive. Accordingly, it is difficult to set the chromatic dispersion zero in a wide wavelength range.
In order to solve this problem, a chromatic dispersion flattened fiber in which chromatic dispersion is approximately zero in a wide wavelength range is used as a transmission path. Further, a dispersion compensating fiber having a negative dispersion slope has been developed, for example, as known by European Conference on Optical Communication '94, pp. 681-684. However, it is difficult to produce these fibers because these fibers are complex in the form of refractive index distribution so as to be not controllable.
On the other hand, a chirped grating has been proposed as means for compensating chromatic dispersion, for example, as known by Optical Fiber Communication Conference '94, postdeadline paper-2, PD2-1 to PD2-4. First, a fiber grating will be described. The fact that the refractive index of a core portion of a Ge-doped core optical fiber is increased when ultraviolet rays of wavelength near 240 nm are radiated onto the Ge-doped core optical fiber is known by Inoue et al, "Generation of Fiber Grating and Application thereof", SHINGAKU-GIHOU, OPE94-5, Institute of Electronics, Information and Communication Engineers of Japan. A periodic refractive index change is formed in the fiber core by using the ultraviolet rays induced refractive index change, by which a diffraction grating can be obtained so that a specific wavelength can be reflected by the diffraction grating.
FIG. 10 is an explanatory view for explaining the chirped grating. In the drawing, the reference numeral 61 designates an optical signal of wavelength .lambda..sub.1 ; 62, an optical signal of wavelength .lambda..sub.2 ; 63, an optical signal of wavelength .lambda..sub.3 ; 64, an optical signal of wavelength .lambda..sub.4 ; and 65, an optical fiber. The relations between the magnitudes of the wavelengths are .lambda..sub.1 &gt;.lambda..sub.2 &gt;.lambda..sub.3 &gt;.lambda..sub.4. The chirped grating operates so that the wavelength reflected by the aforementioned diffraction grating is shifted in the direction of the length of the fiber, that is, chirped. Chromatic dispersion can be compensated by the chirped grating. The optical fiber 65 has the core portion in which the refractive index is changed by the ultraviolet rays induced refractive index change. Respective optical signals 61 to 64 of wavelengths .lambda..sub.1 to .lambda..sub.4 incident to the optical fiber from the left in the drawing are reflected at intermediate portions so as to return to the incident side.
The refractive index change, that is, the period of the grating is designed so as to be gradually reduced from the incident side toward the right so that an optical signal of a longer wavelength is reflected at a position nearer the incident side. Further, by writing the grating so that the percentage of the change of the period of the grating is reduced as the wavelength becomes longer, the dispersion slope can be selected to be negative. Incidentally, because chromatic dispersion is a slope of propagation delay time with respect to wavelength, and the dispersion slope is a slope of the chromatic dispersion, the fact that the dispersion slope is negative means the fact that the dependency of propagation delay time on wavelength is convex upwards.
FIG. 11 is an explanatory view for explaining a WDM transmission method. In the drawing, the reference numeral 71 designates an optical signal transmitter for transmitting an optical signal of wavelength .lambda..sub.1 ; 72, an optical signal transmitter for transmitting an optical signal of wavelength .lambda..sub.2 ; 73, an optical signal transmitter for transmitting an optical signal of wavelength .lambda..sub.3 ; 74, an optical signal transmitter for transmitting an optical signal of wavelength .lambda..sub.4 ; 75, a transmission path; 76, amplifiers; 77, a transmission path; 78, a transmission path; 79, an optical receiver for receiving an optical signal of wavelength .lambda..sub.1 ; 80, an optical receiver for receiving an optical signal of wavelength .lambda..sub.2 ; 81, an optical receiver for receiving an optical signal of wavelength .lambda..sub.3 ; and 82, an optical receiver for receiving an optical signal of wavelength .lambda..sub.4. Now, it is assumed that amplified WDM transmission is of four signal wavelengths, by way of example. In the transmitter side, the optical signal transmitters 71 to 74 of wavelengths .lambda..sub.1, .lambda..sub.2, .lambda..sub.3 and .lambda..sub.4 are connected to one transmission-side transmission path 75 by a multiplexer not shown. The transmission path 75 is connected to the final one amplifier 76 and the receiver side transmission path 78 through one pair of the relay amplifier 76 and the transmission path 77 or a plurality of pairs of the relay amplifiers 76 and the relay transmission paths 77. The transmission path 78 is connected to the optical receivers 79 to 82 of wavelengths .lambda..sub.1, .lambda..sub.2, .lambda..sub.3 and .lambda..sub.4 by a demultiplexer not shown.
Description will be made specifically by using numerical values. The span of the transmission path 77 is about 80 km. In the case where the transmission path 77 is comprised of a 1.3 .mu.m single mode fiber, chromatic dispersion in wavelength of 1550 nm is 17 ps/nm/km, so that the quantity of compensated chromatic dispersion of the overall relay distance is 1360 ps/nm. Even if the amplification wavelength range of the optical fiber amplifier is estimated to be 1550.+-.10 nm, that is, the width of the amplification wavelength range is estimated to be 20 nm, the delay time difference of (1360.times.20=27200 ps=)27.2 ns is required between the optical signal of the longest wavelength and the optical signal of the shortest wavelength in the amplification wavelength range. Consequently, the length of the optical fiber 65 giving the chirped grating shown in FIG. 10 reaches 2.7 m unpractically.
Incidentally, the delay time difference means the propagation time difference between the signal of the shortest wavelength and the signal of the longest wavelength in the wavelength range of the optical signal as a subject. To set the delay time difference to be A[ps], the grating length L[mm] is selected to be L.apprxeq.3.times.10.sup.11 /1.5.times.A.times.10.sup.-12 .times.(1/2)=A.times.10.sup.-1 [mm]. Here, 3.times.10.sup.11 [mm] is the velocity of light in vacuum, 1.5 is the refractive index of glass, and (1/2) is a coefficient obtained by taking into account the round trip of the optical signal.
Accordingly, a method in which gratings of narrow wavelength widths near the respective optical signal wavelengths .lambda..sub.1 to .lambda..sub.4 are produced and the gratings thus produced are arranged is known by the aforementioned Optical Fiber Communication Conference '94, postdeadline paper-2, PD2-1 to PD2-4. In this method, however, there arises a problem that optical signal wavelengths .lambda..sub.1 to .lambda..sub.4 in respective systems have to be known in advance, etc.